Nitinol, a nickel-titanium alloy, is well known in the art for its superb elasticity. Nitinol wire and wire mesh can be permanently formed into a desired shape by a process called training. Training consists of first manually manipulating the trainable material into the desired shape and then heat treating it while holding it firmly in that shape. The heat treatment consists of subjecting the material to a high temperature for a particular amount of time and then cooling it. For nitinol, this high temperature is usually between about 450 and 600° C., and this time usually ranges from a few seconds to three minutes. After the nitinol is subsequently cooled, it will maintain the desired shape.
Methods of heat treating are well known in the art. Heat treating a material requires a uniform rate of heating and cooling to minimize undesired stresses on the material. A method of heat treating and quenching complex geometries without distortion or cracking is taught in U.S. Pat. No. 4,789,410 by Parizek. It teaches the use of two salt baths held at different temperatures into which the material to be treated is submerged at different times.
During the heat treatment, the nitinol must be held firmly in its desired shape, otherwise the shape of the nitinol will warp during the heat treatment. In order to overcome this warping and to provide a firm support for the nitinol during heat treatment, the standard method of forming nitinol into a desired shape is to first create a metal mold that can withstand a high temperature. The mold consists at least of a first piece and a second piece, the first piece able to fit snugly into the second piece, and the two pieces able to lock together. The nitinol in a wire or wire mesh form is then sandwiched between the two pieces and then the pieces are pressed and locked together, thus giving the nitinol the desired shape. The entire mold with the nitinol inside is then subjected to the heating process. For more complicated shapes, more than two pieces may be used, but in each case every part of the nitinol is sandwiched between at least two of these pieces and remains sandwiched during the heating process.
Another method of reducing warpage during the heat treatment is taught in U.S. Pat. No. 6,210,500 by Zurfluh. It teaches a fixture that has porous support walls that engage opposite sides of the material to be heat treated. A liquid heating and cooling media is then flowed through the porous walls to heat treat the material while the support walls minimize warpage of the material.
One problem with conventional methods is that they are not conducive to certain shapes. For example, shapes that have an enclosed volume cannot be easily formed using the conventional methods. This is because once the heat treatment has taken place, it is not possible to remove a solid metal mold from inside. One way to solve this problem is by using a collapsible inside mold, but this method is very expensive and complicated. Another problem with this method is that some complicated shapes are not even conducive to such collapsible molds.
Another way to solve the problem of training materials into complicated shapes is by using a mold that is created from sand bound by a resin binder. The material can then be shaped around the sand mold and heat treated with the sand still inside. After the heat treatment, the sand mold can be broken down and removed, for example, by vibration. A means for processing and reusing sand molds is described in U.S. Pat. No. 5,901,775 by Musschoot et al.
One problem with such a method is that the sand mold is very crude and the resulting trained material must be processed or machined in order to remove the roughness and imperfection resulting from the sand mold. Another problem is that the external part of the mold, made of metal or sand (depending on the application) is bulky and heavy. This results in three further problems: high expense of the external mold; high expense of continually heating and cooling mold materials; high expense of large heat treating baths or ovens and processing equipment; difficulty in achieving a uniform rate of heating and cooling due to the thickness of the external part of the mold. This makes the cooling process and cooling rate harder to control. The cooling rate must be carefully controlled. Nitinol must be cooled to a point at which it retains the desired shape without the support of the mold, but if it is cooled too quickly, it can become unacceptably brittle.